The objective of this proposal is to elucidate the neuroendocrine mechanisms whereby photoperiod controls reproduction in female sheep. It has been proposed that seasonal breeding is caused by photoperiod-induced changes in the negative feedback action of estradiol on LH secretion. However, the mechanisms whereby changes in daylength act on the hypothalamo-pituitary axis to elicit changes in LH secretion remain to be elucidated. Therefore, in these studies, we will investigate whether estradiol exerts its negative feedback action on LH secretion by causing a decrease in secretion of gonadotropin-releasing hormone (GnRH) or by causing a decrease in pituitary response to GnRH, or both. Subsequently, the modulation of this negative feedback mechanism by photoperiod will be examined. In addition, we will attempt to develop a method for direct chronic measurement of GnRH secretion in unanesthetized animals. Finally, the role of the suprachiasmatic nucleus, the putative circadian clock, in transduction of photoperiodic information to the hypothalamo-pituitary axis in sheep will be investigated. Techniques include radioimmunoassays for serum LH and GnRH; controlled delivery of steroid hormones by means of Silastic implants; blood collection; X-radiography; histologic perfusion, fixation, sectioning, and staining; and a variety of surgical procedures, including ovariectomy, and stereotaxic implantation of guide tubes and cannulae. It is expected that the results will provide new insight into the interactions among the environment, the brain and the ovary, which bring about the condition of natural reversible fertility known as seasonal breeding.